A carbon nanotube is a substance whose various industrial applications are expected by a variety of properties due to its ideal one dimensional structure, for example, good electrical conductivity, heat conductivity and a mechanical strength. Improvement in performance and widening of applicability are expected, by controlling a diameter, the layer number and a length of the carbon nanotube.
The carbon nanotube usually has the high graphite structure when the layer number is smaller. It is known that since a single-walled carbon nanotube and a double-walled carbon nanotube have the high graphite structure, they have high properties such as electrical conductivity and heat conductivity. In addition, among carbon nanotubes, since double-walled to quintuple-walled carbon nanotubes having the relatively small layer number have properties of both of a single-walled carbon nanotube and a multi-walled carbon nanotube, they attract an attention as a promising material in a variety of utilities.
As the known process for producing a carbon nanotube, synthesis by a laser ablation method or a chemical vapor deposition method (CVD method) is known. In a method of synthesizing carbon nanotubes by a catalyst chemical vapor deposition method (CCVD method), a catalyst body in which a catalyst metal is supported on a support is used. There are a variety of shapes of the catalyst body such as a powder shape, a gel shape and a plate shape. In respect of a yield of the resulting carbon nanotube relative to an amount of a catalyst to be used, it is good to use a powder-like or aerosol-like support having a great specific surface area and, in respect of easy handling, it is preferred that a plate-like or powder-like support is used. When balance between both of a yield and easy handling is taken into consideration, use of a powder-like support is preferred. In addition, as a material of a support, organic substances and inorganic substances are used, but from respect of easy handling and general versatility, inorganic substances are used in many cases. As an inorganic support, supports having a variety of compositions such as oxide, hydroxide, other metal salt etc. are used.
As a support, from respect of easy removal after synthesis, powder-like magnesium salt and magnesium oxide which can be easily removed only by acid treatment are used in many cases. As an example using magnesium oxide, Patent Documents 1 to 3 are exemplified, but when the method described in Patent Document 1 was merely used, a composition containing carbon nanotubes having a high purity was not obtained. In addition, in the method shown in Patent Document 2, it is necessary to separately produce a catalyst which makes a carbon source to be easily degraded, in addition to a catalyst for synthesizing carbon nanotubes, and the number of steps is increased. In a method of heating a precursor of magnesium oxide to convert it into magnesium oxide (Patent Document 3), it was necessary to make a device, such as rapid heating of an aqueous solution at 650° C. at production of a catalyst body, addition of a foaming agent such as citric acid etc. and combustion in order to increase a surface area of a catalyst body. In addition, since the catalyst body obtained herein is very easily reactive with water and carbon dioxide in the air, and has a very small bulk density, there was a problem on handling, such as easy flying etc.
In addition, in the case of a catalyst obtained by simply mixing and heating a magnesium salt and a catalyst metal salt, without the aforementioned device, it is difficult to obtain a carbon nanotube having a high purity at a good yield.
The carbon nanotube is a material having high electrical conductivity, and can be expected as a transparent electrically conductive material. Upon actual use, in order to attain high electrical conductivity with a smaller amount, it is necessary that the carbon nanotube is uniformly dispersed in a matrix. The dispersing method can be roughly classified into two methods of a method of dispersing by modifying the carbon nanotube itself, and a method of dispersing the carbon nanotube using a dispersant such as a surfactant and a polymer, and since uniform high dispersion is possible while electrical conductivity is maintained, the method using a dispersant is preferably used (Patent Documents 4, 5).
A diameter (outer diameter) of the carbon nanotube is from a few nm to a few hundreds nm and, inter alia, when a carbon nanotube having a small average outer diameter is used, since an electrically conductive network can be formed effectively with a small amount, an electrically conductive composite having high transparency can be obtained. However, a carbon nanotube having a small average outer diameter has a problem that, generally, in purification and dispersing steps, a defect is easily generated on a graphite surface, and electrical conductivity is reduced. In addition, since the carbon nanotube having a small average outer diameter has a strong aggregating force, and is dispersed with difficulty, in order to obtain sufficient dispersibility, an excessive amount of a dispersant is necessary, and a long dispersing time and a strong dispersing force become necessary and, therefore, it was difficult to simply obtain a dispersion having high transparent electrical conductivity.